1. Field of the Invention
This invention relates to labyrinth seal arrangements and, in particular, to a labyrinth seal arrangement having the ability to fit a variety of turbine diameters.
2. Description of the Prior Art
In rotating machinery, for example, large turbo-generators, it is imperative that suitable seal arrangements be provided to prevent the passage of pressurized fluid from a region of higher to lower pressure along an interface between a rotating and stationary element of the machine. Such a necessity arises in a variety of locations within the turbo-machine, for example, between the radially outer surface of the shroud ring attached to the tips of rotating blades and the radially adjacent inner surface of the casing or between the outer surface of the rotating shaft and the radially adjacent inner surface of the turbine casing at the axial points along the shaft where the shaft passes through the casing.
Typical prior art solutions to the sealing problem above outlined utilize a device known as a labyrinth seal. The labyrinth seal disposes a plurality of axially adjacent seal strips which depend from the interior of the stationary casing to block the radial gap between the rotating and stationary members and reduce that gap to a clearance of several thousandths of an inch. There may also be provided on the rotating element deflector seal strips which extend radially outward from the rotating member and are spaced so as to lie axially between the adjacent labyrinth strips. Typical prior art construction requires that radial overlap occur between the labyrinth and deflector strips. An inventive improvement in the sealing arrangement just described wherein deflector seal strips are utilized is that disclosed and claimed in the copending application of C. A. Meyer, Ser. No. 665,347, filed Mar. 9, 1976, assigned to the assignee of the present invention.
With or without the combination of deflector strips however, the purpose of the labyrinth seal is to provide a tortuous path for fluid particles to negotiate in an attempt to pass from a higher to a lower pressure region along the interface between the stationary and rotating member. Typically, the prior art labyrinth seal takes one of two forms. First, the rigid caulked-in seal is secured into a groove machined into the cylinder or stationary element. This arrangement proves costly during manufacturing based on the fact that the final radial clearance is only a few thousandths of an inch and both the rotor shroud and stationary seal are machined to drawing dimensions prior to assembly. It is only possible to determine the accuracy of the small clearance after the rotor is assembled in the cylinder. To correct the radial clearance, if necessary, the large cylinder is disassembled, correctively machined, and returned for final assembly and for final seal clearance confirmation. To restore original seal radial clearance is even more time consuming and costly when the problem is encountered in the field.
The second form of prior art seal is the two-legged springback seal. This consists of a two-labyrinth seal segment, precisely machined to the correct diameter. Basically, since every turbine stage has a different diameter, the seals cannot be interchanged between stages in the event of damage, over-machining or for any other reason. Over-machining, for example, can easily occur due to the fact that the seals are radially customized. As illustrated in the caulked-in seal discussed above, the major assembly must be substantially dismantled for radial seal correction. The radial customizing of the springback seal is to machine the radial legs of the seal in a special portable machine as part of the assembly operation. A short measurement between the rotating and stationary components enables the radial customizing. Some seal arrangements also entrap steam condensate or precipitated contaminants and are thus disadvantageous for use within turbo-machinery.
Representative of the prior art seal arrangements, are U.S. Pat. Nos. 3,694,882, issued to Desmond and assigned to the assignee of the present invention; 3,501,245, issued to Ivanko; 3,082,010, issued to Morley; 2,946,609, and 2,871,038, both issued to Comery; and 2,886,351, issued to Heard. It is noteworthy at this point to appreciate that in referencing prior art patents as background herein, no representation is made that the cited subject matter is the best available prior art.
It is apparent from the foregoing that prior art seal arrangements are disadvantageous for several reasons. Applicants herein disclose a seal arrangement which overcomes all of the above-mentioned disadvantages of the prior art.